The present invention relates to methods of manufacturing piezoelectric products used in the fields of electronics and electro-optics, by deposition of ferroelectric films and, more particularly, to an improved method for electrophoretic deposition (EPD) of ferroelectric films by using a tri-functional additive in EPD suspensions, compositions of suspensions for effecting same, and ferroelectric films formed therefrom.
Ferroelectric materials are used in many applications in electronics and electro-optics. Due to growing demand for miniaturization of electronic products, particularly for micro-electro-mechanical (MEM) devices or systems, there is a complementary growing need for miniaturized piezoelectric elements or piezoelectric films, of thickness on the order of less than 100 microns. Production of thin and thick piezoelectric films, where, in general, thin and thick films are of thickness of less than about 2 microns and greater than about 2 microns, respectively, started in the 1980's. Piezoelectric films are applied in the production of high dot per inch (DPI) ink jet printing nozzle arrays for printing heads, and for functional devices possessing specific electrical, mechanical, and piezoelectric properties or characteristics, such as solid state devices including actuators, micro-machinery, ultrasound transducers, pyroelectric sensors, electro-optic displays, ferroelectric field-effect transistors, high value capacitors, non-volatile memory field-effect transistors, infrared sensors, and optical switches.
It will be appreciated that application of piezoelectric films is an important basis for next century's electronic and electro-optic devices.
Several methods are currently used for manufacturing piezoelectric films and elements by deposition, varying greatly both with respect to methodology and with respect to obtained results. These methods include physical vapor deposition (PVD) such as sputtering, laser ablation and evaporation or molecular beam epitaxy (MBE), chemical vapor deposition (CVD) methods such as metal-organic chemical vapor deposition (MOCVD) and plasma enhanced chemical vapor deposition (PECVD), and wet chemistry methods such as sol-gel processing, electrophoretic deposition, liquid phase epitaxy, tape casting, and slip casting.
Each of these methods has advantages and disadvantages. PVD and CVD deposition methods ordinarily require sophisticated and expensive equipment, making them either undesirable or impracticable for implementation in manufacturing environments. PVD methods involving vacuum techniques have the advantage of being based on dry processes featured by high levels of system purity and cleanliness, translating to relatively high compatibility with fabrication of semiconductor integrated circuits. However, sputtering and MOCVD deposition methods are limited by difficulties in controlling component stoichiometry, especially in multi-component systems, and deposition rates arc low. In these methods, there is also a need for post deposition annealing due to occurrence of internal stresses. The technique of laser ablation may be quick for producing films of usable quality, but yields poor thickness uniformity for deposition areas larger than about 1 cm.sup.2, and is limited by particulate ejection from the target, thereby causing defects in the deposited ferroelectric film.
Simpler and less expensive ferroelectric deposition methods include sol-gel processing, tape casting, slip casting, and electrophoretic deposition (EPD). Sol-gel processing techniques combine the advantages of high compositional control, thin coating capability, and low equipment costs, but sol-gel processing techniques are limited by the occurrence of large volume changes during the deposition process. Sol-gel techniques are useful for thin film production, but are relatively slow for producing thick films having thickness larger than about 1 micron because multiple deposition is required. Tape casting and slip casting are part of wet methods of film deposition, and are used for conventional production of piezoelectric elements of thicknesses of tens to several hundreds of microns, and are not thin film technologies.
Electrophoretic deposition (EPD), a relatively new ferroelectric materials processing technique, in which charged ferroelectric particles, typically submicron in size, dispersed or suspended in a liquid medium are attracted and deposited onto a substrate acting as an electrode of opposite charge, during application of an electric field through the liquid, has been successfully applied to the production of thin, less than 2 microns, and thick, greater than 2 microns, ferroelectric films and elements for the manufacture of piezoelectric devices. Commonly used ferroelectric materials are of the perovskite type, such as lead-zirconate-titanate, PbZrTiO.sub.3 (PZT), lead titanate, PbTiO.sub.3 (PT), and barium titanate, BaTiO.sub.3 (BT).
For better understanding and appreciation of the present invention, an EPD process may be considered to involve two principle stages. The first stage involves electrophoretic movement or migration of the ferroelectric particles through an EPD suspension, and the second stage involves deposition of the ferroelectric particles onto an electrode or treated substrate. Hereinafter, the term `EPD suspension` or `electrophoretic suspension` refers to the combination or suspension of an `EPD liquid medium`, and the ferroelectric particles, with or without addition of one or more liquid or solid phase additives and/or binders. Furthermore, hereinafter, an `EPD liquid medium` refers to a pure solvent, or combination of two or more pure solvents, without addition of any liquid or solid phase additive and/or binder. Ferroelectric particles may be added to an EPD liquid medium either before or after addition of one or more liquid or solid phase additives and/or binders.
Effective electrophoretic deposition of ferroelectric materials is strongly dependent upon using a suitable EPD liquid medium which exhibits three important characteristics of rapidly and uniformly (i) dispersing the ferroelectric particles while preventing particle sedimentation and agglomeration, prior to and during the migration stage of the EPD process (ii) charging the ferroelectric particles, prior to and during the migration stage of the EPD process, and (iii) binding of the ferroelectric particles to the selected substrate and to each other, during the deposition stage of the EPD process. Exhibiting all three characteristics is critical for enabling the manufacture of piezoelectric elements or devices having suitable electrical and piezoelectric properties such as high dielectric constant, and other physico-chemical properties such as uniform film thickness, and high film strength.
Commonly used EPD liquid media include pure, or mixtures of two or more miscible, polar organic solvents such as alcohols, ketones, aldehydes, providing strong ionization effects to the dispersed particles. Typically, ferroelectric particles are initially dispersed in a selected pure solvent, and if it is determined that there is insufficient dispersion, or particle charging, mixtures of varying concentration ratios of two or more solvents are then used for improved particle dispersion and/or charging in the EPD liquid medium. Concentration ratios of two solvents as EPD liquid media, for example, are usually in the range of about 90/10 to 10/90, volume/volume.
An example of electrophoretic deposition of PZT films is described in U.S. Pat. No. 5,462,647, issued to Bhattacharya et al., wherein PZT is dispersed in an aqueous dimethylsulfoxide (DMSO) solution. Electrophoretic deposition of PT and PZT films is described in Japanese Patent Application No. 127439, of Apr. 28, 1995, wherein either a PZT or a PT powder is dispersed in pure acetone.
In both teachings, a main objective is recovery of deposited PZT or PT in powdered form. Extensive elemental analysis was performed for characterizing composition of the recovered PZT or PT powders, but minimal attention was focused on optimization of deposition rate, or of other parameters important during the manufacture of piezoelectric devices, such as density, strength, and shrinkage of the deposited films, especially with respect to effect and influence of EPD suspension characteristics during the migration and deposition stages of the EPD process.
An example of electrophoretic deposition of ferroelectric BT thick films is that reported by Nagai et al., Communications of the American Ceramic Society, January 1993, in the Journal of Am. Ceram. Soc., 76, 1993. BT films 10-70 microns thick were formed from EPD suspensions featuring BT dispersed in a range of mixed ethanol-acetylacetone liquid media. It was observed that colloidal suspensions of BT powder were difficult to deposit from either ethanol or acetylacetone alone, however, upon adding acetylacetone to ethanol, BT films were successfully deposited onto the platinum cathode. The amount of BT deposit, measured as mg BT film per square centimeter of film area, increased with increasing concentration of acetylacetone in ethanol, became independent of the concentration ratio between 20 to 80 volume percent of acetylacetone/ethanol, and gradually decreased until from pure acetylacetone BT was again difficult to deposit. It was proposed that this solvent effect was due to the presence of free protons arising from the aldol reaction between acetylacetone and ethanol.
Addition of a liquid phase additive to the liquid medium of an EPD suspension, where an additive is defined as being of concentration less than 10 volume percent of the EPD suspension, has been done. Examples of additives include acetylacetone, nitric acetate and tartaric acid.
In addition to improving particle dispersion by using an EPD liquid medium including a second or third organic solvent, the main function of a much lower concentration additive in an EPD suspension is to enhance ferroelectric particle dispersion and stabilization of such a suspension via effecting conductivity, pH and viscosity, of the EPD suspension, while preventing particle sedimentation, thereby enhancing migration and deposition stages of the EPD process, ultimately leading to an improved piezoelectric element or device. Depending upon the electrochemistry and relative concentration of a specific additive in an EPD suspension, an additive may also perform a second function as a charging agent of the ferroelectric particles, thereby enhancing particle migration during the migration stage of the electrophoretic process and formation of a deposited film having improved dielectric and piezoelectric properties. Ideally, an additive to an EPD suspension should function in three ways, thereby enhancing all three important characteristics needed for highly effective EPD of ferroelectric films, as described above, including rapid and uniform dispersion of the ferroelectric particles while preventing particle agglomeration, prior to and during the migration stage of the EPD process, charging the ferroelectric particles, and binding of the ferroelectric particles to the selected substrate and to each other, during the deposition stage of the EPD process.
Until now, however, there are teachings of only single or dual functioning additives used for EPD of ferroelectric particles, and no teaching of a tri-functional additive. Enhancement of ferroelectric particle binding characteristics of an EPD suspension is currently accomplished by addition of either a separate binder, or of a dispersing agent which also acts as a binding agent, to the EPD liquid medium or suspension.
In EPD of ferroelectric films, the function of a binder is to improve physical characteristics such as green film density and quality of the deposited film on an electrode or surface activated substrate, immediately following the stage of charged particle migration in the electrophoretic process. However, side effects due to the presence of a binder in an EPD suspension may not be synergetic with important desirable effects of enhancing dispersion and charging of the ferroelectric particles. In currently used EPD suspensions for EPD of ferroelectric films, there often exists a trade off between enhancing dispersion and charging of ferroelectric particles, and enhancing binding of the particles, according to the type and concentration of additives used in a given EPD suspension, and according to the physicochemical interaction between the one or more additives and the selected ferroelectric material.
Electrophoretic deposition of PZT was studied by Sweeney and Whatmore, Ferroelectrics, vol. 187, 57-73, 1996. Films of thickness of 10 microns were deposited from suspensions of PZT in acetone having less than 1 volume percent of nitric acid as an additive, and having less than 1 weight percent of nitrocellulose as a binder. It was determined that the presence of nitric acid additive significantly enhanced PZT particle dispersion and charging, leading to improved characteristics and properties of the deposited PZT films. They observed that the zeta potential of the suspended PZT particles increased with increasing nitric acid concentration in the range of 0.04 ppm to 400 ppm. High values of zeta potential are desirable, since migration velocity or electrophoretic mobility of the ferroelectric PZT particles is directly proportional to zeta potential of the particles in the EPD suspension. They proposed that the nitric acid additive promotes a reaction at the PZT particle surface causing an increase of the electrical charge of the PZT particles in suspension. In this particular case, nitric acid as additive also functions as a charging agent of the suspended PZT particles. In this particular case, nitric acid behaves as a dual functioning additive with respect to enhancing dispersion and charging characteristics of the suspended PZT articles.
As indicated above, the presence of a binder in a suspension of ferroelectric particles can cause undesirable effects during the migration stage of the electrophoretic process, such as a decrease in particle charge and particle migration velocity, translating to a decrease in rate of film deposition. This was confirmed by Sweeney and Whatmore, where they observed that the presence of the nitrocellulose binder had an extremely deleterious effect on the zeta potential of the suspended PZT particles. They proposed that this drop in zeta potential is caused by the nitrocellulose adsorbing onto the particle surface resulting in a significantly large reduction of the potential across the adsorbed surface layer. The overall net effect of using a separate nitrocellulose binder in addition to nitric acid additive in the EPD suspension was to improve binding characteristics of the PZT films, but at a cost of decreasing migration characteristics of the particles through the suspension.
To one of ordinary skill in the art, there is a clearly defined need for an improved method of electrophoretic deposition of ferroelectric films involving the use of a tri-functional additive in EPD suspensions, which eliminates the need for including a separate binder in the suspension, and simultaneously enhances desirable and necessary dispersing, charging, and binding characteristics of suspended ferroelectric particles, ultimately improving manufacture of piezoelectric products, devices, and elements, in a cost effective manner. There is also a clear need, and it would be highly advantageous to have improved compositions for effecting the same, and for having improved ferroelectric films and free-standing products obtained thereof.